Police tend to get upset when they try to arrest you and you refuse to help them.
The cars may hold the inscription, "To Protect and Serve," but the truth is far less noble. The job of the police is to arrest or ticket people who have committed a crime. That is why they are hired, and why they get paid. They don't know you, don't care about you, and if they find that you've committed a crime and do not arrest you, they can be fired or disciplined. The officer may be the nicest guy you've ever met, and he may even chat with you and joke around while he's putting your handcuffs on, but he will arrest you if you give him reason to. A few days ago, officers opened fire on a couple of women driving a truck that looked like one someone they were looking for was driving. No warning, no offer to let them surrender, they just started shooting. Will they be disciplined? More than likely, but that will be all. They will not be prosecuted, arrested, or face charges. Why? Police do not operate under the same sets of laws that you do. They are allowed to stretch and break the laws if they need to in order to arrest you. They can lie, cheat, make stories up, engage in psychological torture if it will get you to admit to a crime. They are the police, and you are the guy behind bars who will not be heard from for the next twenty to thirty years - long after he's retired.
You MUST protect yourself. Remember these phrases if you are stopped and remember the most important one, do NOT answer questions.
Am I being Detained?
If you are being detained, the police have a suspicion that you are someone they are looking for. Every word that comes out of your mouth is being analyzed, recorded, and put in a file to use against you in court. If you tell them you are meeting a friend, and then later say you are meeting a girlfriend - you may have spoke truth, but you have also just damaged your credibility in court. "Officer, can you tell me what he was doing?" "Yes, he was first meeting a friend, then he was meeting a girlfriend. He was lying to us, and that led us to further the investigational questions." Juries will turn on you in a heartbeat if they think you lied.
Being Detained is bad. You may answer a question that means nothing to you and to the officer at the time, then a crime happens days later and you get a return visit that is not as pleasant. You should never answer any question other than your name and address (many states require this) and nothing else. NEVER lie to a police officer. They can lie to you, but you cannot lie to them. Don't believe the lies, and don't talk. And if you never remember anything else here, remember this... NEVER explain yourself. Keep your mouth shut. Why? Every question - even if you don't understand it and even if it is an innocent sounding question, is designed to find a reason to arrest you. That is the only reason they are talking to you - to find a reason to arrest you.
Officer, am I free to go?
This is one that you should know by heart. It is also the answer to a lot of unrelated questions. If the police ask to search your car, be very careful how you answer, but one answer is always good... Officer am I free to go? You are not only saying no to a search, but asking to leave. You can answer every question put to you with this phrase other than your name and address. Some states require an ID, but most do not. Know the laws of your state. If the officers refuse to disengage from you after you say this phrase, then they have a suspicion that you are someone they are looking for. You are in DEEP trouble if they detain you. Remember the magic words - Lawyers Fees! If you are not free to go, then you need the third statement.
Officer, I wish to remain silent.
Now, this one is tricky. The cops may still ask you questions, may still yell at you, plead with you, cajole you, and even promise to not arrest you if you answer some more questions. The response should always be silence. Do not talk. Don't speak. Parley Not. Open-eth your Mouth-eth not-eth. To the police, you are a criminal at this point, and it is likely to them that they are going to send you to jail, and they need just a little more evidence to arrest you. They are after you to provide them that evidence. They will lie, make things up, threaten you with jail if you don't talk, tell you that you can help yourself if you answer, tell you that they will help you get lesser charges, and even tell you that you will be charged with as many felonies as they can think up if you don't answer. Being in a 6'X3' cell where the lights are never off and you are filmed using the bathroom is totally up to you at this point. If you talk, you will go to that jail cell. If you keep your mouth shut, you offer them nothing more than what they had before. So do yourself a favor, keep your mouth shut.
Oh, if you keep talking, then you are what they call, "Engaging the officers," and as long as you are engaging them, you prolong the stop.